Remove Layer suffix from LayerType variants

[aleokdev]

Done to improve readability:

// Previously
match &layer.layer_type() {
    LayerType::TileLayer(l) => do_thing(),
    LayerType::ObjectLayer(l) => do_thing_2(),
    _ => ()
}
// Now
match &layer.layer_type() {
    LayerType::Tile(l) => do_thing(),
    LayerType::Object(l) => do_thing_2(),
    _ => ()
}

[bjorn]

Since we're renaming anyway, would it make sense to use Tiles and Objects, since these layers contain multiples, whereas an Image layer does not, and Group already indicates multiple?